Integrated circuit systems are rapidly increasing in device count and operating speed. Each of these attributes tends to increase power requirements directly, while both together cause power requirements to increase exponentially. Therefore, dealing with power requirements associated with integrated circuit systems is becoming a much more critical design arena than before. One such management technique involves separating the integrated circuits that require full power all of the time to perform their intended tasks from those that may use reduced power to perform some of their intended tasks at least some of the time.
Supplying and removing electrical current, either partially or completely, from a block of circuitry may be controlled by header or footer transistors. The header transistor forms a controllable switch between a positive power supply and a sub-circuit block. Similarly, the footer transistor forms a controllable switch between a negative power supply and the sub-circuit block. Activation of the header or footer transistor allows a virtual supply to be connected to the sub-circuit block. Often, both header and footer transistors are employed to provide both positive and negative virtual supplies concurrently when the block of sub-circuitry is active. Similarly, header and footer transistors are used to reduce current to blocks of sub-circuitry during their inactive modes of operation.
A conventional approach to providing an operating virtual voltage to the sub-circuit uses the drain of a conducting header or footer MOS transistor wherein its source and body region are connected to a supply voltage. Then, the forward voltage drop of a separate, external junction diode that is parallel-connected to the MOS transistor is used to provide a standby virtual voltage for the sub-circuit when the MOS transistor is not conducting. This approach is bulky, thereby adding to die size and an increased risk of defects, which reduces chip yield.
Accordingly, what is needed in the art is a more efficient way of providing virtual operating and standby voltages.